And unlike Lady Cora - an American import like McGovern herself, who has been based in Britain for the past 20 years - Mrs Thatcham is decidedly English. McGovern's own voice sounds closer to the mid-Atlantic tones of Lady Cora.
If interest in the film, which debuted earlier this year at New York's Tribeca Film Festival, is because of McGovern's Downton connections, that was never planned. The movie was shot after the first season of Downton had been made but before it was broadcast.
It's the feature debut of Donald Rice and based on a 1932 novella by Julia Strachey (niece of The Bloomsbury Group's Lytton Strachey) about Dolly Thatcham (Felicity Jones) whose wedding day happiness is undermined by the arrival of ex-boyfriend Joseph who wants her to call the whole thing off and not be forced into the life her mother has planned out for her. Meanwhile McGovern's mother of the bride quietly fumes about the disruption in between dealing with her guests.
So it's not as lighthearted a movie as its title might suggest, more a drawing-room melodrama.
"I feel it is a gentle comedy of manners. It goes a different way to most movies you see particularly ones about young love. What I thought when I read it was it seems like a real dialectic between two ways of looking at love - one of them is the very immature romantic way of looking at love as represented by the boy who crashes the wedding.
"The other approach to love is a more middle-aged compromised version, full of the human foibles we have to put up with when we are talking about a creation of a family and all those things that it ultimately leads to, if it lasts.
"I think nobody should go to it expecting a romantic comedy in that sense of the word. It isn't that and that is what makes it great. If you are looking for a typical commercial romantic comedy you are not going to find it in that movie. If you are looking for something a little more subtle and to me possibly like real life that is what it will give you."
The film is a modest one-house affair - "low budget on a scale that was beyond even my experience of low budget" she laughs- and possibly cost less than an episode of Downton.
But it returns the Chicago-born Los Angeles-raised 51-year-old McGovern to the big screen where she started out in the early 80s in movies like Ordinary People, Ragtime (for which she was Oscar nominated), and Once Upon a Time in America.
Marrying English then-theatre director Simon Curtis in 1992, McGovern shifted to the UK where the couple raised two daughters - she jokes Lady Cora's various looks of exasperation at her offspring had plenty of practise at home.
In Britain she's toiled away in theatre and various television roles until Downton came along and became an international, award-winning hit. Funny how things work out ...
"Yeah, I've been here for 20 years so I've been hacking away for a long time. It seemed like the worst career idea in the world when I first moved. It seems exactly the opposite of what anybody who wanted to conquer the world should do because I was leaving a career I had worked very hard to establish in America."
The forthcoming third season, which reportedly begins in 1920 with the news that the Grantham family fortune has been lost, has her excited though understandably circumspect about what she can say.
"I think it's the best series yet in terms of the story. I am really excited about it. I just think because you have this luxury of having characters that the audience have known for a certain amount of time you can go more deeply in ways with them that you don't normally have the opportunity to do, and so I am loving all that. I just relish it. It's marvellous."
In season three, veteran American actress Shirley MacLaine turns up as Martha Levinson, Lady Cora's presumably moneyed mother.
"I like to say, and it's absolutely true, that when she walked on the set I suddenly understood so much more about my character than I ever had. And what's more, my character became three times more interesting. I was nothing but the beneficiary of it."
As for fans of the show, many of whom didn't like the hurried second series as much as the first: "I don't think anyone will be disappointed."
Lowdown
Who: Elizabeth McGovern, best known as Downton Abbey's Lady Cora
What: The film Cheerful Weather for the Wedding
When: At selected cinemas now.
- TimeOut